Organic-inorganic composite materials are finding increasing importance due to their extraordinary properties which arise from the synergism between the properties of the organic and inorganic components and interfacial regions. The tremendous possibilities of different combinations between organic and inorganic species enable the fabrication of nanocomposite materials in various shapes having unique properties which can not be obtained by traditional composite materials.
A critical challenge in the design of such organic-inorganic systems is control of the mixing between the two dissimilar species, which determinates the homogeneity of the final material or product. The formation of interpenetrating networks (IPNs) between organic and inorganic moieties (or species), starting from their perspective liquid forms, is one of the most promising ways to attack this challenge. The simultaneous synthesis of the networks can result in a substantially homogeneous material. A major problem during this process, however, arises from the different stabilities of organic and inorganic species. While inorganic systems are thermally quite stable and are often formed at high temperature, most organic ingredients have an upper temperature limit below 300° C. Therefore, the synthesis of organic-inorganic composite systems requires a strategy wherein the formation of the components is well-suited to each other; in this case, the organic species dictate the use of a low-temperature formation procedure. For this reason, milder reactions are required for the formation of the inorganic network. An ideal procedure for the generation of such composite materials is the sol-gel process. Sol-gel processes allow formation of the composite materials made of inorganic and organic components.
Sol-gel processes have been known as low temperature routes to making inorganic glass precursors in comparison to traditional SiO2 glass formation at temperatures above 1400° C. Using the sol-gel processes, the condensation of reactive hydrolyzed metal alkoxides can occur in liquid form over a temperature range between 25 and 60° C. The sol-gel procedure is two step processes during which metal alkoxides are hydrolyzed to form metal hydroxides, which in turn condense to form a three-dimensional network.
Acid or base catalysts may be used for hydrolysis process. By varying the catalysts, significant effects on gelation time, bulk and apparent density, and volume shrinkage during drying are observed (Brinker C. J., Scherer G. W. Sol-gel Science. San Diego: Academic Press; 1990).
In another aspect, the sol-gel process through the organic saturated acid solvolysis of different metal alkoxides is also known (Pope E. J. A., Mackenzie J. D. Journal of Non-Crystalline Solids, 87, 1986, 185-198). The reaction of tetramethoxysilane (TMOS) or tetraethoxysilane (TEOS) with a variety of organic carboxylic acids at room temperature produces a gel over a time period between a few minutes to a few days.
Solvolysis with acetic acid can be found in the literature (Bekiari V., Lianos P. Langmuir, 14, 1998, 3459; Stathatos E., Lianos P., Lavrencic-Stangar U. L., Orel B. Adv. Mater., 14, 2002, 354). In particular, it was found that acetic acid solvolysis proceeds by two-step reaction. In the first step, a silica ester (CH3—COOSi—) is formed while in the second step SiOH forms in reaction with ethanol. From the latter, by inorganic polycondensation, a —O—Si—O— network is created, which provides the gelling agent. A simplified reaction scheme derived from FTIR data is represented by the following reactions:
Solvolysis Ester Formation:—SiOC2H5+CH3COOH→—SiCH3COO+C2H5OH>—SiOH+CH3COOC2H5 Inorganic Polycondensation—(SiOH)n→n(SiO2)+mH2O
Additionally, it is known that hydrolysis and gelation with the use of strong inorganic acids like HCl is so rapid that it leads to the higher inhomogeneity of final product as compared with the case of using weaker organic acids for solvolysis. (Stathatos E., Lianos P., Lavrencic-Stangar U. L., Orel B. Adv. Mater., 14, 2002, 354).
Although silicon alkoxides are probably most studied, other metals for use in sol-gel processes include titanium, tin, zirconium, cerium and aluminum (Kikelbick G. Prog. Polim. Sci., 28, 2003, 83-114). Therefore the inorganic part of the composite material is not limited to SiO2.
Polymer-titanium composite materials present peculiar interest as high refractive index compositions. Long-Hua Lee et al. (Long-Hua Lee and Wen-Chang Chen. Chem. Mater., 13, 2001, 1137-1142) reported the synthesis of high-refractive index trialkoxysilane-capped PMMA-titanium hybrid optical thin films being prepared by an in situ sol-gel process. The acrylic monomers used were methyl methacrylate (MMA) and 3-(trimethoxysilyl)propyl methacrylate (TMSPM). Titanium (IV) n-butoxide was used for the preparation of the titanium network. The FTIR and DSC results indicate the successful bonding between the organic and inorganic moieties and the good dispersion of the PMMA segments in the titanium network. The off-resonant refractive indexes of the prepared thin films at 633 nm were in the range of 1.505-1.867 as the titanium content linearly increased from 2.9 to 70.7% Wt.
The synthesis of composite organic-inorganic composite materials can be conducted with the addition of different monomers or prepolymers into the reacting mixtures. The use of monomers is limited by their compatibility with starting reagents and by the use of hydrophilic medium (polar water solution) for a synthesis. For this reason, hydrophilic monomers such as HEMA or vinylpyrrolidone (VP) are usually used. More hydrophobic monomers like styrene and its analogues can be preliminary copolymerized with such precursors of the inorganic phase as TMSPM or triethoxyvinylsilane (TEVS) or styrylethyltrimethoxysilane (SEMS) (Wei Y et al. J. Mater Res., 8, 1993, 1143-1152; Feng Q. et al. J. Mater. Chem., 10, 2000, 2490-2494). The functionalized polymers are hydrolyzed and co-condensed with TEOS and/or other precursors of the inorganic phase for the formation of an inorganic network via acid-catalyzed sol-gel route. The lack of well-defined glass-transition temperatures for the polymers in the silica matrix revealed that the polymer chains are uniformly distributed in the materials. So those hybrid materials have an excellent optical transparency.
Nevertheless, considering all the advantages of the sol-gel process, it is necessary to take into account a significant disadvantage, especially in the case of the synthesis of monolithic hybrid composite samples. Once formed, the gelled metal oxide network must be dried, requiring the removal of water excess, cosolvent(s) and liberated alcohol. It is this requisite drying process that effectively prevents the practical and reproducible synthesis of monoliths or thick films with dimensions greater than a few millimeters, because sol-gel organic-inorganic composites tend to shrink, crack and shatter. Cracking can be minimized in several ways, including very slow, controlled drying of the composite composition over the course of weeks or months, by increasing the average pore size through the introduction of colloidal silica seed particles, by adding surfactants, by supercritical drying or by the addition of special reagents.
One of the major obstacles to the widespread application of sol-gel techniques is the fact that this drying process is accompanied by extraordinary shrinkage of the solid inorganic phase. Related to the volume fraction of volatiles removed, this shrinkage is routinely of the order 50-70% (Novak B. M., Ellsworth M. W. Mater. Sci. and Eng., A162, 1993, 257-264). Shrinkage on this scale precludes many molding applications and can introduce a high degree of stress in sol-gel monolithic composites. In the view of this fact, it is clear that the resulting weight yield of solid product of sol-gel process is limited up to 20-30%. Even in the case of solvolysis of metal alkoxides with pure organic acids, without water and cosolvent(s), there are the liquid by-products (acid esters and water as the polycondensation by-product). As a result, the shrinkage exists in all modification of sol-gel techniques.
In view of the problems and disadvantages associated with existing organic-inorganic nanocomposites, it would be desirable to provide new organic-inorganic nanocomposite materials and new methods of making organic-inorganic nanocomposite materials.